Carbon nanotubes allow ballistic conduction in which electrons are transported without being scattered, and show excellent electrical conductivity (low electrical resistance) and high current density tolerance (high electromigration tolerance). Therefore, carbon nanotubes are expected as wiring material for the next-generation of semiconductor devices to replace Cu wiring that is currently used in the mainstream.
A known carbon nanotube growth technique is a plasma CVD method of exciting and decomposing hydrocarbon molecules or the like as raw material using high-energy plasma and growing a carbon nanotube by reacting active species with a catalytic metal. An example of a carbon nanotube film forming method using the plasma CVD method is disclosed in Patent Document 1 (Japanese Patent Laid-open Publication No. 2007-252970) which suggests a method of forming a catalytic layer made of a transition metal such as Ni, Fe, and Co on a substrate and forming a carbon nanotube film thereon by a plasma CVD method using a carbon-containing gas and a hydrogen gas at a treatment temperature of 600 degrees C. The method of Patent Document 1 includes activating the surface of a catalytic metal by supplying radicals in the plasma using the carbon-containing gas and the hydrogen gas to the surface of the catalytic metal, thereby preventing a reduction in catalytic activity of the catalytic metal due to oxidation of the surface thereof when the catalytic metal is converted into fine particles. However, since the method of Patent Document 1 requires heating up to approximately 600 degrees C., even in the plasma CVD method, the substrate itself and a material film formed on the substrate must have a heat resistance corresponding to the heating temperature. Therefore, it is difficult to apply the method of Patent Document 1 to, for example, a plastic substrate or the like.
There is also a known thermal CVD method of thermally decomposing hydrocarbon molecules of raw material on the catalytic metal surface to grow a carbon nanotube. As a carbon nanotube film forming method using the thermal CVD method, for example, Patent Document 2 (Japanese Patent Laid-open Publication No. 2007-261867) suggests a method of forming a carbon nanotube film on a substrate, on which a fine particle catalyst is formed, by a thermal CVD method using a hydrocarbon gas as a raw material at a temperature of 800 to 1000 degrees C. The method of Patent Document 2 includes: a fractional process of converting a catalytic metal thin film such as a Fe thin film formed on a substrate into a granular catalyst by heating the thin film to 800 to 1000 degrees C. in an oxygen atmosphere (for example, in an atmospheric atmosphere) to melt the thin film and then cooling the thin film; and a re-fractional process of converting the granular catalyst into fine particles by heating the granular catalyst to 800 to 1000 degrees C. in the oxygen atmosphere (for example, in an atmospheric atmosphere). However, in the method of Patent Document 2, since the substrate supporting the catalytic metal is heated to a high temperature of 800 degrees C. or more, which may cause diffusion of impurities in the substrate, the substrate requires a large thermal budget. Therefore, it is difficult to apply the method of Patent Document 2 to a semiconductor device manufacturing process.
In addition, Patent Document 3 (Japanese Patent Laid-open Publication No. 2011-68513) suggests a method of performing an oxygen plasma treatment on a metal catalytic layer, activating the surface of the metal catalytic layer by performing a hydrogen-containing plasma treatment, and then forming a carbon nanotube film on the activated metal catalytic layer using a plasma CVD method.
In order to use a carbon nanotube as wiring material of semiconductor devices or the like, the carbon nanotube is required to have i) high density, ii) high vertical orientation, and iii) high quality. Improvement of crystalline orientation by raising the carbon nanotube growth temperature is effective to achieve “iii) high quality”, and “ii) high vertical orientation” can be effectively achieved by raising the carbon nanotube density so that thin carbon nanotubes can be mutually supported. That is to say, “i) high density” is important to achieve “ii) high vertical orientation” and “iii) high quality”.
However, even if catalytic metal fine particles serving as nucleus of carbon nanotube formation are disposed at high density on the substrate, when the activation ratio of the catalyst is small, the carbon nanotube cannot be formed at high density. The suggestion of Patent Document 3 is excellent in that the activation ratio of the catalytic metal fine particles is increased by performing the hydrogen plasma treatment after the oxygen plasma treatment. However, it is believed that, if the activation ratio of the catalytic metal fine particles is further improved, the carbon nanotube can be formed at a higher density while high vertical orientation can be also achieved.